Interface ITopicViews
This feature allows a client session to manage topic views.
Inherited Members
Namespace: PushTechnology.ClientInterface.Client.Features
Assembly: Diffusion.Client.dll
Syntax
public interface ITopicViews : IFeature
Remarks
A topic view maps one part of the server's topic tree to another. It dynamically creates a set of reference topics from a set of source topics, based on a declarative topic view specification. The capabilities of topic views range from simple mirroring of topics within the topic tree to advanced capabilities that include publication of partial values, expanding a single topic value into many topics and throttling the rate of publication.
Each reference topic has a single source topic and has the same topic type as its source topic. Reference topics are read-only (they cannot be updated), nor can they be created or removed directly. Otherwise, they behave just like standard topics. A client session can subscribe to a reference topic, and can fetch the reference topic's current value if it has one.
The source topics of a topic view are defined by a topic selector. One or more reference topics are created for each source topic, according to the topic view. If a source topic is removed, reference topics that are derived from it will automatically be removed. If a topic is added that matches the source topic selector of a topic view, corresponding reference topics will be created. Removing a topic view will remove all of its reference topics.
Topic view specifications
The following is a simple topic view specification
that mirrors all topics below the path a
to reference topics below the path b
.
map ?a// to b/<path(1)>
A topic view with this specification
will map a source topic at the path a/x/y/z
to a reference topic at the path b/x/y/z
.
The specification is simple,
so the reference topic will exactly mirror the source topic.
A general topic view specification comprises several parts:
- The source topic clause identifies the source topics.
- The path mapping clause determines how reference topic paths are derived from the source topic paths, and when expanding to more than one reference topic, from where the values are obtained.
- The optional topic property mapping clause determines how reference topic properties are derived from source topic properties.
- The optional value mapping clause determines how reference topic values are derived from source topic or expanded values.
- The optional throttle clause constrains the rate at which each reference topic is updated when its source topic is updated.
Source topic clause
The source topic clause begins with the
map
keyword
and is followed by a topic selector.
These topic selectors follow the same parsing rules
as other topic selectors.
When evaluating a topic view, topics in the topic tree that match the source topic selector are considered, with the following exceptions:
- Topics created through the Publisher API;
- ROUTING topics.
Both SLAVE and reference topics are valid source topics. In particular, chaining of topic views is supported; that is, a reference topic created by one topic view can be the source topic of another topic view. Additionally, a reference topic can be the master topic of a slave topic, or the source topic of a routing topic subscription.
Tip
Prefer topic views to slave topics. Individual topics can be mirrored by creating a slave topic, but maintaining slave topics for a branch of the topic tree quickly becomes tedious. A topic view will maintain such a branch automatically, and provides more sophisticated mapping options.
Path mapping clause
The paths of reference topics are derived from the source topic according to the topic view path mapping. The path mapping allows the source topic path and the value of the source topic to determine the path of the reference topic. In addition the path mapping can include expand directives which allow objects and arrays in JSON source topic values to be expanded to produce many reference topics.
A path mapping clause begins with the
to
keyword
and is followed by a path mapping template.
A path mapping template is a topic path
with embedded directives.
Directives are evaluated when creating the topic reference
and substituted into the topic path.
Directives are delimited by angle brackets
<
,
>
and consist of the name of the directive
and a list of parameters.
The parameter list is comma-separated
and surrounded by parentheses ((
, )
).
The following mapping directives are supported:
Source path directives extract a portion of the source path
and are parameterized by the index
of the start part of the source path
and the number of parts to include.
The number of parts parameter is optional
– if it is missing,
the selection extends to the end of the source path.
The syntax is
<path(start, number)>
,
or <path(start)>
when the number of parts parameter is omitted.
For example,
given the source path
a/b/c/d
,
the source path directive
<path(1, 2)>
is mapped to the reference topic path
b/c
,
and the source path directive
<path(2)>
is mapped to the reference topic path
c/d
.
Topic property mapping clause
The ITopicSpecification of a reference topic is derived from the topic specification of the source topics. A reference topic has the same topic type as its source topic.
The topic properties of a reference topic are derived from the source topic. Some topic properties can be tuned using the optional topic property mapping clause. The following table describes the behavior for each topic property.
Source topic property | Reference topic specification default | Can be set by topic property mapping? | Notes |
---|---|---|---|
Compression | Copied from source topic specification | Yes | |
Conflation | Copied from source topic specification | Yes | |
DontRetainValue | Copied from source topic specification | Yes | |
Owner | Not set | No | |
Persistent | Not set | No | Reference topics are not persisted. Topic views are persisted, so a reference topic will be recreated on server restart if its source is persistent. |
Priority | Copied from source topic specification | Yes | |
PublishValuesOnly | Copied from source topic specification | Yes | |
Removal | Not set | No | Reference topics cannot be removed directly. |
Schema | Copied from source topic specification | No | A RECORD_V2 reference topic has the same schema as its source topic. |
SlaveMasterTopic | Not set | No | If a reference topic has a slave topic as its source topic, it indirectly references the slave's master topic. |
TidyOnUnsubscribe | Copied from source topic specification | Yes | |
TimeSeriesEventValueType | Copied from source topic specification | No | A TIME_SERIES reference topic has the same value type as its source topic. |
TimeSeriesRetainedRange | Copied from source topic specification | Yes, with restrictions | A topic property mapping cannot increase the time series retained range by overriding the TimeSeriesRetainedRange property. The retained range of a reference time series topic will be constrained to be no greater than that of its source topic. |
TimeSeriesSubscriptionRange | Copied from source topic specification | Yes | |
ValidateValues | Not set | No | A reference topic reflects updates to its source topic. It cannot reject updates. |
A topic property mapping clause begins with the keywords
with properties
and consists of a comma-separated list
of topic property keys and values,
each separated by a colon.
For example,
the following topic view specification
maps all topics below the path
a
to reference topics below the path
b
,
and disables both conflation
and compression for the reference topics.
map ?a// to b/<path(1)> with properties 'CONFLATION':'off', 'COMPRESSION':'false'
Topic value mapping
By default, a reference topic's value is a copy of the source topic value, or part of the source value produced by an expand path mapping directive. For JSON source topics, the optional topic value mapping clause can be applied to extract part of the source value, or to further refine the value produced by the expand directive.
A topic value mapping begins with the keyword
as
and is followed by a value directive.
A value directive is delimited by angle brackets
(<
, >
),
and consists of the
value
keywords and a single JSON pointer parameter.
The JSON pointer selects the part of the source value to copy.
For example, given a source value of
{
"account" : "1234",
"balance" : { "amount" : 12.57, "currency" : "USD" }
}
and the value mapping clause
as <value(/balance)>
,
the reference topic value will be
{
"amount" : 12.57,
"currency" : "USD"
}
Value mappings that follow expand directives apply to the expanded value and not the source topic value.
Topic value mappings only alter the reference topic value;
only the path mapping determines whether a reference topic
should exist. If the topic value mapping's JSON pointer fails
to select anything from the source topic value, the reference
topic will have the JSON value null
.
Topic value mappings are often used with path value mappings to avoid repeating information in the path and the value. For example:
map ?accounts// to balances/<scalar(/account)> as <value(/balance)>
Value mappings that follow expand directives apply to the expanded value and not the source topic value.
Throttle clause
The optional throttle clause can be used to constrain the rate at which a reference topic is updated when its source topic is updated. The primary application of a throttle clause is to restrict the number of updates sent to reference topic subscribers, reducing network utilization or the processing each subscriber must do. Throttling also restricts the rate at which client sessions can observe changes to reference topic values using the fetch API.
The throttle clause has the form
throttle to X updates every period
,
where X is a positive integer,
and period is a positive integer
followed by a time unit which is one of
seconds
,
minutes
,
or hours
.
For example,
the following topic view specification
maps all topics below the path
a
to reference topics below the path
b
,
but updates the value of each reference topic
at most twice every five seconds:
map ?a// to b/<path(1)> throttle to 2 updates every 5 seconds
To improve readability,
the throttling clause allows
1 update
as an alternative to
1 updates
,
and every second
as an alternative to
every 1 seconds
(and so on, for other time units).
For example,
the following topic view specification
maps all topics below the path
a
to reference topics below the path
b
,
but updates the value of each reference topic
at most once every hour:
map ?a// to b/<path(1)> throttle to 1 update every minute
The throttle clause is only applied when a source topic is updated more frequently than the configured rate. If a source topic is updated less frequently, updates are passed on unconstrained. If the rate is exceeded, a reference topic will not be updated again until the configured period has expired. At this time, the reference topic will be updated based on the source topic updates that happened in the interim, and a single value will be published. Thus, a throttle clause provides topic-scoped conflation.
The throttle clause is ignored for time series topics because time series updates do not support efficient conflation. Updates to source time series topics are passed on immediately to the corresponding reference topics, regardless of any throttle clause.
Escaping and quoting special characters
Each part of a topic view expression
has characters with special significance.
Source topic clauses
and path mapping clauses
are delimited by white space.
Directives in path
and topic property mapping clauses
are delimited by the
<
and >
characters,
and each directive parameter is terminated by
,
or
)
.
Topic property mapping
clauses are delimited by white space,
and the :
and ,
characters.
Sometimes a topic view must refer to
or generate topics with paths
that contains special characters,
or use a JSON pointer containing special characters.
The escape sequence
\x
can be used to literally insert any character
x
,
with a one exception:
\/
cannot be used in path fragments
since the path delimiter
/
is always significant.
Here is an example topic view expression
containing escape sequences.
It maps the topic path
a topic
a reference topic with the path
another topic
.
map a\ topic to another\ topic
Here is an example with a source value directive
that uses the JSON pointer
/x()/y
to extract the target path from the source value.
The
)
character in the JSON pointer must be escaped
so it is not treated as the end of the parameter list.
map ?a// to <scalar(/x(\)/y)>
To insert
\
,
the escape sequence
\\
must be used.
There is no need to escape white space
in JSON pointers directive parameters.
However,
white space is significant.
For example,
the following expressions have different topic value mapping clauses
since the JSON pointer in the second expression is
"/x "
;
that is,
it has a trailing space:
map a to b as <value(/x)>
map a to b as <value(/x )>
Instead of using escape sequences,
white space characters can be included in source topic clauses
and path mapping clauses using quotes.
A clause is quoted by wrapping it in single quote
('
)
or double quote
("
)
characters.
For example:
map "a topic" to "another topic"
Within a quoted clause, quotes of the same type must be escaped:
map 'alice\'s topic' to 'bob\'s topic'
For consistency, the values in topic property mapping clauses can be escaped or quoted. However, there is no need to do so because none of the valid values for the mappable properties contain special characters.
Dealing with topic path conflicts
Reference topics have a lower priority than normal topics created through the API, including replicas of normal topics created by topic replication or fan-out. A reference topic will only be created if no topic or reference topic is already bound to its derived topic path.
Topic views have a precedence based on order of creation. If two topic views define mappings the same topic path, the earliest-created topic view will create a reference topic. If a topic view is updated, it retains its original precedence.
Topic view persistence and replication
Reference topics are neither replicated nor persisted. They are created and removed based on their source topics. However, topic views are replicated and persisted. A server that restarts will restore topic views during recovery. Each topic view will then create reference topics based on the source topics that have been recovered.
The server records all changes to topic views in a persistent store. Topic views are restored if the server is started.
If a server belongs to a cluster, topic views will be replicated to each server in the cluster. Topic views are evaluated locally within a server. Replicated topic views that select non-replicated source topics can create different reference topics on each server in the cluster.
Access control
The following access control restrictions are applied:
- To list the topic views, a session needs the READ_TOPIC_VIEWS global permission.
- To create, replace, or remove a topic view, a session needs the MODIFY_TOPIC_VIEWS global permission and SELECT_TOPIC permission for the path prefix of the source topic selector.
- Each topic view records the principal and security roles of the session that created it as the topic view security context. When a topic view is evaluated, this security context is used to constrain the creation of reference topics. A reference topic will only be created if the security context has READ_TOPIC permission for the source topic path, and MODIFY_TOPIC permission for the reference topic path. The topic view security context is copied from the creating session at the time the topic view is created or replaced, and is persisted with the topic view. The topic view security context is not updated if the roles associated with the session are changed.
Note
This interface does not require user implementation and is only used to hide implementation details.
Added in 6.3.
Examples
An instance of this feature can be obtained by calling TopicViews.
// session is a previously obtained ISession instance.
var topicViews = session.TopicViews;
Or it can be obtained by calling Topics.
// session is a previously obtained ISession instance.
var topics = session.Topics;
Methods
CreateTopicViewAsync(String, String)
Create a new named topic view.
Declaration
Task<ITopicView> CreateTopicViewAsync(string name, string specification)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
String | specification | The specification of the topic view. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
If a view with the same name already exists the new view will update the existing view.
If the operation completes successfully, the Task
result will be a
ITopicView instance.
Exceptions
Type | Condition |
---|---|
InvalidTopicViewException | The supplied specification is invalid. Thrown by the returned |
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission or
appropriate path prefix permissions. Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRepartitionException | The cluster was repartitioning. Thrown by the returned |
See Also
CreateTopicViewAsync(String, String, CancellationToken)
Create a new named topic view.
Declaration
Task<ITopicView> CreateTopicViewAsync(string name, string specification, CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
String | specification | The specification of the topic view. |
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<ITopicView> | The |
Remarks
If a view with the same name already exists the new view will update the existing view.
If the operation completes successfully, the Task
result will be a
ITopicView instance.
Exceptions
Type | Condition |
---|---|
InvalidTopicViewException | The supplied specification is invalid. Thrown by the returned |
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission or
appropriate path prefix permissions. Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRepartitionException | The cluster was repartitioning. Thrown by the returned |
ListTopicViewsAsync()
List all of the topic views that have been created.
Declaration
Task<IReadOnlyCollection<ITopicView>> ListTopicViewsAsync()
Returns
Type | Description |
---|---|
Task<IReadOnlyCollection<ITopicView>> | The |
Remarks
If the operation completes successfully, the Task
result will be a
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
See Also
ListTopicViewsAsync(CancellationToken)
List all of the topic views that have been created.
Declaration
Task<IReadOnlyCollection<ITopicView>> ListTopicViewsAsync(CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<IReadOnlyCollection<ITopicView>> | The |
Remarks
If the operation completes successfully, the Task
result will be a
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have READ_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
RemoveTopicViewAsync(String)
Remove a named topic view if it exists.
Declaration
Task<object> RemoveTopicViewAsync(string name)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
Returns
Type | Description |
---|---|
Task<Object> | The |
Remarks
If the named view does not exist the Task
will complete successfully.
If the task completes successfully, the Task
result will be null
.
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRepartitionException | The cluster was repartitioning. Thrown by the returned |
See Also
RemoveTopicViewAsync(String, CancellationToken)
Remove a named topic view if it exists.
Declaration
Task<object> RemoveTopicViewAsync(string name, CancellationToken cancellationToken)
Parameters
Type | Name | Description |
---|---|---|
String | name | The name of the view. |
CancellationToken | cancellationToken | The cancellation token used to cancel the current operation. |
Returns
Type | Description |
---|---|
Task<Object> | The |
Remarks
If the named view does not exist the Task
will complete successfully.
If the task completes successfully, the Task
result will be null
.
Exceptions
Type | Condition |
---|---|
SessionSecurityException | The calling session does not have MODIFY_TOPIC_VIEWS permission.
Thrown by the returned |
SessionClosedException | The calling session is closed. Thrown by the returned |
ClusterRepartitionException | The cluster was repartitioning. Thrown by the returned |